Conventionally, there is known an electric valve timing adjusting device that adjusts valve timing of an engine using rotation torque of a motor. In the electric valve timing adjusting device, a driving side rotation body to which torque of a crankshaft is transmitted, and a driven side rotation body that transmits torque to a camshaft are connected together via a planetary gear. By changing a rotation speed of a planetary rotation body relative to the driving side rotation body by the motor of the valve timing adjusting device, a phase of the driven side rotation body relative to the driving side rotation body is changed, so that opening or closing timing of a valve which is opened or closed by the camshaft is changed.
The valve timing adjusting device has a stopper function of restricting a relative rotation between the driving side rotation body and the driven side rotation body. In JP-A-2012-237203, for example, there is described a valve timing adjusting device in which, when a driven side rotation body rotates relative to a driving side rotation body, a projecting part that is formed radially outward of the driven side rotation body can be in contact with a projecting part that is formed radially inward of the driving side rotation body.
In the valve timing adjusting device described in. JP-A-2012-237203, when opening or closing timing of a valve is set at the most retarded angle or the most advanced angle, a side wall of the projecting part of the driven side rotation body is brought into contact with a side wall of the projecting part of the driving side rotation body. The inertia force of rotational movement of the driven side rotation body is transmitted to the driving side rotation body, The acting force in a radially outward direction acts on the driving side rotation body via bearings of the driving side rotation body and the driven side rotation body. The driving side rotation body includes a sprocket on which, for example, a timing belt is wound, engine torque being transmitted to the timing belt, and an outer gear having relatively high rigidity which is located radially inward of the sprocket and is in engagement with a planetary gear. When the side wall of the projecting part of the driving side rotation body collides with the side wall of the projecting part of the driven side rotation body, the reaction force against the acting force which is caused by the inertia force of rotational movement of the driven side rotation body is applied to the sprocket from the outer gear. This reaction force is applied in a radially outward direction of the sprocket. Accordingly, the sprocket is required to have strength against the acting force and the reaction force applied in the radially outward direction. As a result, the sprocket is increased in size, so that the size of the valve timing adjusting device increases.